This invention relates to an optical disk apparatus which records video data on, and retrieve the video data from, a spinning disk by illuminating the disk with a laser beam.
An optical disk apparatus of this type is known as a record/playback system of a high recording density. The optical disk apparatus under ongoing development use disks having a higher photosensitivity so as to record video data at a high speed or by using a lowenergy laser beam. A variety of techniques have been proposed not only to raise the photosensitivity of a recording layer of a disk but also to suppress the dissipation of the optical energy concentrated in the recording layer.
With some of the known optical disk apparatus an intense laser beam is used to record data on a disk and a somewhat less intense laser beam is used to retrieve the data from the disk. Even if a less intense beam is applied on the disk to retrieve the data, an extremely large optical energy will be concentrated at a portion of the disk unless the disk is spinning. The concentrated optical energy would deform the recording layer or, in the worst case, break the recording layer. If the portion of the disk is a recorded portion, the data recorded therein will inevitably be destroyed.
Most optical disk apparatus use a laser beam having a diameter of about one micron. The optical energy applied on a disk is therefore extremely large. In other words, the optical energy is much concentrated on the disk in both time and space. In addition, the laser beam, which is generated by amplifyting light waves about 10.sup.5 times, will detach or destroy the retinas if it enters the eyes of a user. In most of the existing optical disk apparatus a disk takes a horizontal position because it can be more easily placed or detached than in case it takes other positions. The lower surface of the disk placed in the horizontal position is illuminated with a laser beam, whereby data are recorded or retrieved. A playback-only disk stores data in its aluminum recording layer, and record/playback disk has a sandwich structure comprised of thin metal layers. Thus, as long as the disk of either type is attached to the apparatus and put in the horizontal position, the laser beam cannot leak from the apparatus. When the disk is removed from the apparatus, however, the beam may leak and may enter the eyes of the user. This would be a very dangerous situation.